Numerous current measuring devices and magnetometers have been developed based upon the Faraday effect. The Faraday effect causes the plane of polarization of a polarized beam of light passing through a transparent substance exhibiting the Faraday effect to rotate from the plane of polarization of the incident light by an amount proportional to the magnetic field passing through the substance parallel to the optical axis of the beam of light.
U.S. Pat. Nos. 3,324,393, 3,419,802, 3,502,978, 3,581,202, 3,590,374, 3,597,683, 3,605,013, 3,693,082, 3,708,747, 3,746,983, 3,978,334, 3,980,949, 4,070,620, 4,070,622, 4,112,367, 4,232,264, 4,243,936, 4,255,018, 4,348,587, 4,363,061, 4,370,612, 4,428,017, 4,516,073, 4,529,875, 4,531,092, 4,539,519, 4,539,521, 4,540,937, 4,563,639, 4,563,646, 4,564,754, 4,578,639, 4,581,579, 4,584,470, 4,612,500, 4,613,811, 4,631,402, 4,683,421, 4,698,497, 4,745,357, and 4,755,665, disclose current or magnetic field sensors based upon the Faraday effect.
Magneto-optic materials exhibiting the Faraday effect have been developed and are commercially available which have a substrate of gadolinium gallium garnet which is coated with a layer of yttrium iron garnet.
In a current sensor or magnetometer based on the Faraday effect, transmission loss variations in the light path to and from the material exhibiting the Faraday effect can cause a sensor system to lose calibration. The transmission loss variations can result from, for example, demating and then remating optical connectors. In addition, variations in the light beam intensity can also cause such a sensor system to lose calibration.
Prior art Faraday effect devices have made numerous attempts to compensate for losses and/or variations in light intensity which would affect the measurements being attempted with the Faraday device. For example, see U.S. Pat. Nos. 4,540,937, 4,531,092, 4,539,521, 4,613,811, and 4,658,497 cited above which illustrate various prior art techniques for correcting or compensating for errors introduced into measurements being conducted with systems utilizing Faraday effect materials as a result of losses due to the optical transmission medium, variations in the light intensity, etc. However, none of these prior art devices attempt to eliminate the effects of optical transmission loss and light beam intensity variations in Faraday effect devices by utilizing a dual channel approach to the processing of signals representing the measured quantity of current flowing in a conductor where an AC/DC ratio calculation is conducted in each channel and the resulting ratios are mathematically combined to isolate the desired AC/DC components of the measured current. U.S. Pat. No. 4,755,665, cited above, discloses a system whereby two incident light beams are processed in separate channels to determine an average AC/DC ratio for each light beam and the resulting ratios are then provided to a divider circuit to compute a ratio of the first average value to the second average value.
A single channel processing approach has been disclosed in Kyuma et al, "Fiber Optic Measuring System For Electric Current by using a Magneto-Optic Sensor" IEEE Journal of Quantum Electronics, Vol. QE-18, No 10, (October, 1982), pp. 1619-1623. The Kyuma et al method is able to provide a single output indicative of AC current flowing in a conductor. The system of Kyuma et al cancels errors in the single output due to variations of the light source intensity, the transmission loss of fibers, the optical connector loss, or the insertion loss of the connector with the aid of a divider which normalizes an AC output with a DC output. However, the resulting single output in this approach is corrupted by a DC component in the measured field.